Method and apparatus for grinding by the use of grinding bodies subjected to centrifugal force



Aprll 15, 1952 N. AHLMANN 2,592,994

METHOD AND APPARATUS FOR GRINDING BY THE USE OF GRINDING BODIES SUBJECTED TO CENTRIFUGAL FORCE Filed July 3, 1947 3 Sheets-Sheet l CYCZO/VE INYENTOR Mwa aflmm ATTORNEYS FillL'EFF Aprll 15, 1952 AHLMANN 2,592,994

METHOD AND APPARATUS FOR GRINDING BY THE USE OF GRINDING BODIES SUBJECTED TO CENTRIFUGAL FORCE Filed July 5, 1947 3 Sheets-Sheet 2 INVENTOR Z/ k162i. M6 441: 1:.

B Y [QM MKMMM ATTORNEYS April 15, 1952 N. AHLMANN METHOD AND APPARATUS FOR GRINDING BY THE USE OF GRINDING BODIES SUBJECTED T0 CENTRIFUGAL FORCE Filed July 5, 1947 25 Sheets-Sheet 3 ATTORNEYS Patented Apr. 15, 1952 UNITED STATES PATENT OFFICE IVIETHOD AND APPARATUS FOR GRINDING BY THE USE OF GRINDING BODIES SUB- JECTED T CENTRIFUGAL FORCE Nikolai Ahlmann, Copenhagen, Denmark,

assignor to F. L. Smidth & 00., New York, N. Y., a corporation of New Jersey Section 1, Public Law 690, August 8, 1946 Patent expires May 28, 1962 13 Claims. 1

This invention relates to the grinding of material by means of freely movable grinding bodies and is concerned more particularly with a novel method and apparatus for grinding by means of grinding bodies, which perform their function by reason of being subjected to centrifugal force.

It has been common practice heretofore to crush and grind materials in containers by means of grinding bodies caused to act by gravity, and such an operation is carried on in a ball or tube mill, in which the rotation of the mill causes the grinding bodies to be lifted and then dropped upon the material. In such mills, small grinding bodies cannot be used to advantage, because their pounding action resulting from gravity alone is not sufficient to crush the material efficiently, and the higher grinding pressures desirable in some instances cannot be obtained even with grinding bodies of substantial size.

The present invention is directed to the provision of a method and apparatus for grinding by the use of grinding bodies, which overcomes the disadvantages above mentioned and makes it possible to utilize the bodies more efiioiently than when they are subjected to gravity action alone. In the practice of the new method, the material to be ground and a charge of grinding bodies are introduced into a vessel of circular cross-section, within which is mounted an agitator having a plurality of agitating members or blades extending generally lengthwise of the vessel and lying close to the wall thereof. In the practice of the method, either the vessel or the agitator is rotated at high speed on the axis of the vessel and, as a result, the grinding bodies and the material are subjected to centrifugal force and accumulate in a layer covering the inner surface of the vessel. In this layer, the

, grinding bodies are caused to move relatively to one another and thereby perform their grinding function.

The grinding operation according to the new method is preferably continuous and the grinding bodies and the material are circulated and the finished ground material is separated and removed from the circulating stream. The circulation may be wholly within the vessel or partly inside and partly outside the vessel, the separating operation being performed by any of the usual means. If desired, the finished material can be removed from the vessel by an air or gas current passed through the vessel and during the grinding, the material may be kept cool preferably by cooling the vessel. The

grinding may be either wet or dry.

The new method affords various advantages over those heretofore used, as, for example, the grinding may be carried on under controlled conditions, so that the pressure exerted by the grinding bodies is not excessive and does not result in waste of power. In addition, it is possible to obtain much higher grinding pressures than those available when the grinding bodies act under the influence of gravity alone. Accordingly, grinding bodies of very small dimensions can be used where appropriate and, in each grinding operation, it is possible to choose grinding bodies of the size that is most economical for the grinding of the particular material to the desired fineness. In some instances, it is possible to use grinding bodies of a size as small as 1 mm. and obtain the desired'grinding pressure.

In carrying on the new method, with the ves-.- sel stationary and the agitator rotating, for ex"- ample, the material to be ground and the grinding bodies are distributed and maintained on the inner surface of the container in the form of a layer by the agitator and those parts of the layer nearest the container wall move at a. very low speed, while those parts of the layer nearest the blades of the agitator move at a speed approximating that of the blades. As the variation in the speed of the parts of the layer decreases gradually through the layer from the high to the low value, the grinding bodies within the layer move relatively. The same situation prevails, when the agitator is stationary and the container rotating, or when both the agitator and container are rotating, but at dilferent speeds. The relative movement of the grinding bodies may also be achieved by scraping the layer wholly or in part from the vessel wall, the displaced portion of the layer then returning to the wall beyond the scrapers.

In the practice ofv the invention, the speed of rotation of the container or the agitator is so high that the acceleration by centrifugal force 3 tion with an inlet for material to be ground and an outlet for the ground material. An agitator is mounted centrally within the vessel and includes members adjacent the inner surface of the vessel, the spacing of the members from the vessel wall depending on the size of the grinding bodies. Thus, if the grinding bodies are of substantial size, the members of the agitator lie close to the wall, so that jamming is avoided,

but, if smaller grinding bodies are used, themembers preferably lie at a'distance from the wall about three times the cross-section of a body. The inner surface of the vessel and the surface of the agitating members may either be smooth or provided with grooves, projections, recesses, or the like to prevent slippage of the grinding bodies thereon. The agitator members may extend parallel to the axis of the vessel .or be inclined to the axis or of helical form and they may be either continuous or interrupted.

Similarly, the vessel wall may be provided with projections of inclined or helical form, so that the material to be ground and the grinding bodies will move through the vessel during grinding.

For a better understanding of the invention, reference may be made to the accompanying drawing, in which form of the new vertical grinding apparatus;

Fig. 3 is a'vertical sectional view of another form of the apparatus;

Fig. 4 is a side elevational view of the apparatus of Fig. 3 with parts broken away;

Fig. 5 is a sectional view on the line 55 of Fi 3;

Fig. 6 is a vertical sectional view. on the line 6-5 of Fig. '7 of another form of the apparatus, including a vessel having its axis vertical;

Fig. 7 is a sectional view on the line 1'| of Fig. 6;

Fig. 8 is a vertical sectional view of another form of the apparatus, including a vessel having its axis vertical;

' Fig. 9 is a sectional view on the line 9-9 of Fig. 8;

Fig. 10 is a sectional view on the line l9l0 of Fig. 9; and

Figs. 11 and 12 are vertical sectional views of different forms of the new apparatus, including a vessel having its axis horizontal.

The apparatus in the form illustrated in Fig. 1 includes a vessel of cylindrical form mounted with its axis vertical and provided near its lower end with an inlet 21 for admission of the material to be ground. A shaft 22 extends axially through the vessel and is mounted for 1'0- tation in bearings '23, 24. The shaft forms part of an agitator, which includes agitating blades 25 of helical form attached to the shaft by radial arms 26. The shaft is provided with a pul- 1 ley 21, by which it can be rotated at high speed. At its upper end, the vessel is provided with a screen 28 enclosed within a hood 29, which closes the upper end of the vessel and is provided with an outlet 39 for ground material. A frusto-conical partition member 3! is supported within the vessel below the upper end thereof on bolts 32 and the upper ends of the blades 25 extend into the partition member.

In the operation of the apparatus above de- 11 scribed, the material to be ground and the grinding bodies are introduced into the vessel at its this layer, the grinding bodies are moved relatively to one another and the layer is moved upwardly along the container wall by the action of blades 25. As the layer traverses the screen 28, the ground material passes through the screen and leaves through outlet 30. Above the level of the partition member 3|, the grinding bodies and insufficiently ground material drop down intothe partition member 3| and are directed thereby down through the central part of the vessel to the lower end, where they are picked up by the blades and again distributed along the vessel wall.

The apparatus illustrated in Fig. 2 includes a cylindrical vessel 34 mounted with its axis vertical and provided at its lower end with an inlet 35 for admission of grindin bodies and material to be ground. An agitator, consisting of shaft 36 and blades 31 carried by radial arms 38 extending from the shaft, is mounted for retation on the axis of the vessel. A casing 39 having an outlet 49 surrounds the vessel near its upper end and that part of the vessel wall within the casing is formed with tangential slots 4|. The shaft carries a plurality of radial vanes 42 near its upper end and the top of the vessel is provided with an outlet 43, which may be connected to a cyclone separator 44. Air is withdrawn from the separator by a fan 45 and introduced into casing 39 through inlet 49. A circular pipe 46 encircles the vessel near the upper end thereof and has openings through which cooling water can be directed upon the outer wall of the vessel, the water being collected in a trough 41 at the bottom of the vessel.

In the operation of the Fig. 2 apparatus, the material to be ground and the grinding bodies are formed into a layer on the inner surface of vessel 34 by the action of the agitator members 31 and the layer is advanced upwardly along the vessel wall. Air supplied by fan 45 enters the upper end of the vessel through casing 39 and slots 4! and air with entrained ground material passes between the vanes 42 and out through outlet 43. Oversize material is separated from the air stream by the vanes and drops to the bottom of the vessel to be distributed along the vessel wall by the agitator blades. The air stream with entrained fine material enters the separator 44, from which the air is withdrawn by fan 45. The separated ground material leaves the separator through the bottom outlet therefrom.

That form of the new apparatus shown in Figs. 3, 4 and 5 includes a cylindrical vessel 48 mounted with its axis vertical and having an inlet 49 for admission of material and a tangentially disposed air inlet 59 at its top. The vessel also has an outlet 5| at the center of its top wall. The bottom wall 52 of the vessel has a central portion of frusto-conical shape and a shaft 53 extends through the bottom wall axially thereof. Within the vessel, the shaft is provided with radial arms 54 in upper and lower series, the ends of the arms of each series being connected by a ring 55. A plurality of blades 56 extend between the rings and are inclined to the axis of the vessel. The blades lie close to the surface of the inner wall of the vessel.

In the operation of the apparatus illustrated in Figs. 3-5, inclusive, the material and grinding bodies introduced into the vessel through inlet 49 are maintained in a layer against the inner surface of vessel 48 by the action of rotating blades 55 and the layer is caused to rise through the vessel by the blades. The blades terminate in a plane somewhat below the top wall of the vessel and the material leaving the tops of the blades lies in the path of the air under pressure introduced through the tangential inlet 58. The sufllciently ground material is carried off with the air through outlet 5| and the coarse material and grinding bodies fall to the bottom of the vessel.

The apparatus shown in Figs. 6 and '7 includes a vessel 5'! of circular cross-section supported on a vertical tubular shaft 58, which operates in a bearing 59 and is provided with a gear 68, meshingwith a pinion BI on a driven shaft 62. The

top of the vessel is closed by a stationary hood 63, which has an inlet 64 for admission of material and an inlet 65 for air under pressure. A stationary shaft 66 extends through the .hood and at its lower end carries radial arms 61, which extend close to the inner wall of the vessel, and shorter radial arms 88. The free ends of adjacent arms v8?, 68 are connected by a scraper 68, which lies close to the inner wall of the vessel and extends lengthwise thereof.

In the operation of the apparatus of Figs. 6, '7 the material to be ground and grinding bodies are introduced into the vessel through inlet 64 and are formed centrifugally into a layer 18 on the inner wall of the vessel by the high speed rotation of the latter. During rotation of the vessel, part of the material in the layer, is removed therefrom by the action of scrapers 89 and travels across the face of each scraper to rejoin the layer beyond the scraper. Air under pressure enters the vessel through inlet 65 and leaves through hollow shaft 58, carrying the sufficiently ground material with it.

The apparatus illustrated in Figs. 8 to 10, inclusive, includes a vessel 18 of cylindrical form with its axis vertical and having an inlet II for material to be ground at its lower end. The vessel is subdivided into a plurality of compartments by partitions I2 and each partition has a central opening surrounded by a frustoconical flange (3. A partition 14 of similar form is mounted on the bottom of the vessel. Each partition I2 is provided with a plurality of openings I5 defined in part by'an upwardly sloping wall IS. A shaft 'Il extends .through the vessel from top to bottom and carries a driving pulley I8 at its upper end. Radial arms I9, 88 extend .from the shaft in upper and lower series in each compartment and the ends of the arms in the series are connected by rings 8|, 82. Agitator blades 83, connect the pair of rings within each compartment and lie close to the inner wall of the vessel. The blades are inclined to the axis of the vessel. The vessel I8 has an outlet 84 for the material and grinding bodies at its upper end and the material and bodies issuing from the outlet are deposited upon a shaking screen 85. Ground material passing through the screen is collected in a hopper 86 and led off through an outlet 87. Oversize material and grinding bodies drop off the top of the screen into a hopper 88 and are returned therefrom to the vessel through inlet II. 1

In the operation of the apparatus shown in Figs. 8-10, inclusive, the material to be ground and grinding bodies entering the lowest com- .partment within the vessel 18 are formed into a layer 89 on the inner wall of the comparte ment by blades 83. The layer is raised by the action of blades and the material and bodies pass through openings I5 into the next higher compartment. The upward travel of the layer continues through the compartments successively, until the layer reaches the outlet 84 and the material and grinding bodies pass through outlet 84 to screen 85. The partitions I3, 14 prevent accumulation of the material and bodies on the partitions I2 and the bottom wall of the vessel.

The apparatus shown in Fig. 11 includes a cylindrical vessel 98 having its axis horizontal and provided with a hollow trunnion 9| mounted in bearings 82 and encircled by a gear 93 driven by a pinion 94 on a drive shaft 95. One end of the vessel is open and lies within a stationary casing 96, through which extends a shaft 9! mounted in bearings 98 and driven by any suitable means. Radial arms 99 extend from the shaft within casing 88 and carry agitator blades I88 at their free ends. The open end of the vessel is'provided with aninternal flange I8! and the casing has an outlet I82..

In theoperation of the Fig. 11 apparatus the vessel 98 is rotated at a high speed and shaft 91 is rotated in the-same direction at a lower speed. The material and grinding bodies introduced through trunnion 9| into vessel 98 form a layer I83 on the inner wall of the vessel. As additionalmaterial is fed into the vessel, the contents thereof overflow flange IN and pass through outlet I82 into a separator I88, from which the sufficiently ground material leaves through outlet I85. The oversize material and grinding bodies leave'the separator through outlet I 86 and are returned into vessel 98 through trunnion 9I.

The apparatus of Fig. 12 includes a vessel I81 of cylindrical form having its axis horizontal and provided at one end with an opening I88,

into which extends the end of a spout I89,

through which material and grinding bodies can be introduced into the vessel. At its opposite end, the vessel is provided with an internal flange vI I8 defining an opening, and this end of the vessel lies within a casing I I I having an outlet H2. A shaft II3 extends through the vessel and is supported for rotation in bearings I I4 and driven through a coupling H5. The shaft carries a plurality of radial arms I I6 having blades JI'I at their outer, ends lying adjacent the inner wall of the vessel.

Material and grinding bodies fed into the vessel I81 through opening I 88 are formed into a layer II8 on the inner wall of the vessel by the action of the blades l I! moving at high speed withshaft I I3. Ground material and grinding bodies overflowing flange II8 enter casing II I and leave through outlet II 2. From the outlet, the material and bodies may pass through a separator similar to separator I84 and the sufficiently ground material may be removed, while the oversize material and grinding bodies may be returned .to spout 189, as in the apparatus shown in Fig. 11.

I claim:

l. A grinding apparatus, which comprisesa vessel of circular cross-section having an inlet for material to be ground and an outlet for ground material, the inlet and outlet lying at opposite ends of the vessel, a charge of -freely movable grinding bodies within the vessel, the amount of the charge being 'suchthat the grinding bodies 'form'only a relatively thin layer on the inner adjacent the inner surface of the vessel and being adapted to engage and cause a relative movement of the bodies and material within and throughout the layer and to propel the material and bodies from the inlet toward the outlet.

2. A grinding apparatus, which comprises a vessel of circular cross-section having an inlet for material to be ground and an outlet for ground material, the inletlying at the lower and the outlet lying at the upper end of the vessel, the vessel being mounted with its axis vertical, a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface ofthe vessel, when distributed thereover, a plurality of agitating members mounted within the vessel about the axis thereof and extending at an angle to the axis and generally lengthwise of the vessel, substantially from the inlet to the outlet and means for causing relative movement of the vessel and members about the axis of the vessel at a speed effective to distribute the material and bodies in a relatively thin layer covering the entire inner surface of the vessel, the members lying adjacent the inner surface of the vessel and being adapted to engage and cause a relative movement of the bodies and material within and throughout the layer and to propel the bodies and material upwardly along the surface of the vessel toward the outlet.

a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface of the vessel, when distributed thereover, an agitator mounted within the vessel for rotation on the axis thereof and including a plurality of members extending generally lengthwise of the vessel substantially from the inlet to the outlet and adjacent the inner surface of the vessel, and means for rotating the agitator at a speed effective to cause the members to maintain the bodies and material in arelatively thin layer covering the entire inner surface of the vessel, the members causing relative movement of the bodies and material within and throughout the layer and propelling the bodies and material from the inlet toward the outlet.

4. A grinding apparatus, which comprises a vessel of circular cross-section having an inlet for material to be ground and an outlet for ground material, the vessel being mounted with its axis vertical, and with the inlet at its lower and the outlet'at its upper end a charge of freely movable grinding bodies within the vessel, the amount of the change being such that the grinding bodies form only a relatively thin layer on the inner surface of the vvessel, when distributed thereover, an agitator mounted within the vessel for rotation on the axis thereof and including a plurality of members extending generally lengthwise of the vessel substantially from the inlet to the outlet and at an angle to the axis thereof, and means for rotating the agitator at a speed effective to cause the members to maintain the bodies and material in a relatively thin layer covering the entire inner surface of the vessel, to advance the layer upwardly through the vessel and to cause relative movement of the bodies and material within and throughout the vessel.

5. A grinding apparatus, which comprises a vessel of circular cross-section mounted with its axis vertical and having an inlet for material to be ground near its lower end and an outlet for ground material near its upper end, a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface of the vessel, when distributed thereover, an agitator mounted within the vessel for rotation on the axis thereof and including a central shaft, radial arms on the shaft, and a plurality of members mounted on the outer ends of the arms and extending generally lengthwise of the vessel substantially from the inlet to the outlet and at an angle to the axis thereof, means for rotating the agitator at a speed effective to cause the members to maintain the bodies and material in a relatively thin layer covering the entire inner surface of the vessel, the members causing the layer to move upwardly through the vessel and producing relative movement of the bodies and material within and throughout the layer, and means within the upper end of the vessel for directing insufliciently ground material and grinding bodies downwardly to the bottom of the vessel inwardly from the members.

6. A grinding apparatus, which comprises a vessel of circular cross-section mounted with its axis vertical and having an inlet for material to be ground near its lower end and an inlet for air and an outlet for ground material near its upper end, a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface of the vessel, when distributed thereover, an agitator mounted within the vessel for rotation on the axis thereof and including a plurality of members extending generally lengthwise of the vessel substantially from the inlet to the outlet and adjacent the inner surface of the vessel, and means for rotating the agitator at a speed effective to cause the members to maintain the bodies and material in a relatively thin layer covering the entire inner surface of the vessel, to cause relative movement of the bodies and material within the layer, and to advance the layer upwardly through the vessel.

7. A grinding apparatus, which comprises a vessel of circular cross-section having an inlet for material to be ground at one end and an outlet for ground material at the other end, the vessel being mounted with its axis vertical, a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface of the vessel, when distributed thereover, a plu-v ralityof stationary agitator members mounted within the vessel, the members extending generally vertically substantially from the inlet to the outlet and lying adjacent the inner surface of the vessel, and means for rotating the vessel on its axis at a speed sufiicient to cause the material and grinding bodies to form a rela tively thin layer covering the entire inner surface of the vessel, the members engaging the layer and causing relative movement of the material and grinding bodies within and throughout the layer and propelling the material and bodies vertically along the vessel wall. a

8. A grinding apparatus, which comprises a vessel of circular cross-section mounted with its axis vertical and having an inlet for material to be-ground near its lower end and an outlet for ground material near its upper end, partitions within the vessel, subdividing it into a vertical series of compartments, each partition having openings near its periphery, a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface of the vessel, when distributed thereover, an agitator within the vessel mounted for rotation on the axis of the vessel and including members in each compartment extending generally lengthwise of the vessel and lying adjacent the inner surface of the vessel, and means for rotating the agitator at a speed such that the members form the material and grinding bodies into a relatively thin layer covering the entire inner surface of the vessel, the members lying at an angle to the axis of the vessel and causing the layer to advance upwardly through the compartments in succession and the bodies and material 'tomove relatively to one another within the layer.

9. A grinding apparatus which comprises a vessel of circular cross-section mounted with its axis horizontal and having an inlet for material to be ground and an outlet for ground material, the inlet and outlet lying at opposite ends of the vessel, a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface of the vessel, when distributed thereover, an agitator mounted within the vessel and including a plurality of members disposed about the axis of the vessel and lying close to the inner surface thereof, the members extending generally lengthwise of the vessel substantially from the inlet to the outlet, and means for causing relative rotary movement of the vessel and agitator on the axis of the vessel at a speed such that the material and bodies are maintained centrifugally in a relatively thin layer covering the entire inner surface of the vessel, the members causing relative movement of the material and bodies within and throughout the layer and propelling the material and bodies from the inlet to the outlet.

10. A grinding apparatus which comprises a vessel of circular cross-section having an inlet for material to be ground and an outlet for ground material, the inlet and outlet lying at opposite ends of the vessel, a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface of the vessel, when distributed thereover, an agitator mounted within the vessel and including a plurality of members disposed about the axis of the vessel and extending generally lengthwise thereof substantially from the inlet to the outlet, means for rotating the'vessel at a speed such that the material and bodies are maintained in a relatively thin layer against the entire inner surface of the vessel, and means for rotating the agitator at a speed different from that of the vessel, the members lying adjacent the inner surface of the vessel and causing relative movement of the material and bodies within the layer.

11. A grinding apparatus, which comprises a vessel of circular cross-section having an inlet for material to be ground and an outletfor ground material, the inlet lying near the lower and the-outlet lying near the upper end of the vessel, a charge of freely movable grinding bodies within the vessel, the amount of the charge being such that the grinding bodies form only a relatively thin layer on the inner surface of the vessel, when distributed thereover, a plurality of agitating members mounted within the vessel about the axis thereof and extending generally lengthwise of the vessel substantially from the inlet to the outlet, means for causing relative movement of the vessel and members about the axis of the vessel at a speed effective to distribute the material and bodies in a relatively thin layer covering the entire inner surface of the vessel, the members lying adjacent the inner surface of the vessel and being adapted to engage and cause relative movement of the bodies and material within and throughout the layer and to propel the bodies and material from the inlet to the outlet, and means for cooling the vessel during the operation of the apparatus.

12. A method of grinding material by means of freely movable grinding bodies, which comprises subjecting the material and bodies to centrifugal force while confining the material and bodies to cause them, under the influence of the centrifugal force, to form a thin layer of V substantially cylindrical form, repeatedly engaging and displacing within the layer the material and bodies forming the inner portion only of the layer to agitate the material and bodies, while said agitated material and bodies are held within the layer by centrifugal force and the outer portion of the layer is continuously held in cylindrical form by such force, continuously removing the material from the layer at one end thereof, and continuously adding material at the other end of the layer at substantially the same rate that the material is removed from the layer.

13. A method of grinding material by means of freely movable grinding bodies, which comprises subjecting the material and bodies to centrifugal force While confining the material and bodies to cause them, under the influence of the centrifugal force, to form a thin layer of substantially cylindrical form, repeatedly engaging and displacing within the layer and material and bodies forming the inner portion only of the layer throughout substantially the entire length of the layer to agitate the material and bodies, while said agitated material and bodies are held within the layer by centrifugal force and the outer portion of the layer is continuously held in cylindrical form by such force, continuously removing material from the layer at one end thereof, and continuously adding material at the other end of the layer at substantially the same rate that'the material is removed from the layer.

NIKOLAI .AHLMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS.

Number Number 10 Number Name Date Balletto May 4, 1926 Wuensch Apr. 25, 1933 Runyan Aug. 27, 1935 Frisch May 19, 1936 Kiesskalt Aug. 4, 1942 Lykken Dec. 8, 1942 FOREIGN PATENTS Country Date Germany July 13, 1908 Sweden Oct. 31, 1917 Germany Oct. 8, 1924 Great Britain Feb. 12, 1925 Great Britain Oct. 9, 1930 Germany Feb. 4, 1937 Germany Oct. 14, 1938 Denmark Aug. 27, 1945 

